The use of radiofrequency electrodes inserted into the body is well known for creating heat lesions, i.e. volumes of destruction of tissue. Heretofore, such lesions have been restricted to functional therapy or pain therapy, meaning that the electrode is inserted into neurological tissue or other tissue approximating nerves so as to heat the tissue by radiofrequency energy dissipation heating and thus to destroy a neurological structure that is the cause of a pain or motor problem. The literature has a large number of examples from many decades, and the equipment and applications are exemplified by the products of Radionics, Inc. in Burlington, Mass. Radionics makes radiofrequency generators (rf generators) and accessory rf electrodes for such therapeutic lesion generation. Recently, reports of use of Radionics equipment for destruction of tumors in the liver have been reported (Buscurini et al.). This involved insertion of an electrode made by Radionics with a hemispherical point and exposed conductive tip into tumors in the liver for the purpose of complete heating destruction by radiofrequency current (rf ablation) of a tumor. The electrode had a fixed tip length of about 10 millimeters and was inserted into the tumor under real time ultrasonic control. The electrode was placed within the tumor volume, and the rf heating done at an empirical level of temperature and radiofrequency generator voltage applied to the electrode. The temperature was measured by a temperature sensor installed within the tip of the rf electrode.
Tumors to be ablated may have great variability in their shape and size. They also can occur in or near critical structures such as the brain, where avoidance of the critical structures, both in terms of the entry tract of the electrode and in terms of the heating ablation zone, must be carefully considered. Heretofore, no method of using three-dimensional pre-planning has been done to implement tumor ablation with radiofrequency electrodes, nor has there been an attempt to select an appropriate tip length of the exposed rf electrode which is optimal to the approach direction and size of the tumor for that given probe tract. Thus, one of the objectives of the present invention is a method and associated apparatus to make such optimization possible.
Hyperthermia is a well known technique. This has typically been implemented by insertion of radiofrequency or microwave electrodes into a tumor volume so as to heat the tumor volume and sometimes nearby normal tissue to sub- or marginally lethal temperatures, and to combine such marginally lethal temperatures with doses of ionizing radiation. The combined effect of marginally elevated temperature and radiation is to selectively destroy tumor cells but spare normal cells, since the latter have the ability to withstand the heat and radiation process much better. This is the hyperthermia principle. It is radically different from the concept of heating a tumor volume to lethal temperatures over the entire tumor volume with the explicit purpose of simply killing all cells, usually without exception within the lethal or ablation isotherm volume. In the case of the present invention, we are describing an ablation volume, and not necessarily describing adjunct radiation therapy, as in the case of the hyperthermia principle. The use of adjunctive radiation therapy may be applied for other reasons in conjunction with the present invention, such as to treat outlying normal tissue that has not been substantially affected by the ablation process or to boost the kill process. The hyperthermia principle also makes use of fractionation or multiple exposures of heat and radiation to attempt to destroy the tumor cells during their cell cycle and spare the normal cells by the same cyclical cell division process. The present rf ablation technique deals typically with total destruction within the target tissue volume. Thus, there is a substantial difference between the hyperthermia method of the past and the present invention of interventive rf tumor ablation.
Part of the present invention is both method and apparatus for implementing the rf tumor ablation. Radionics, Inc. has a variety of electrodes that have been used for various functional or pain procedures, as mentioned above. These electrodes have had various shapes and structure. For example, the GSK Gildenberg Stereotactic Kit contains an electrode which has an insulated cannula and an uninsulated radiofrequency electrode that can be inserted within the cannula such that the length of exposure of the radiofrequency tip can be varied prior to insertion of the electrode or after insertion of the electrode into the body. Radionics also has the RRE Ray Rhizotomy Electrode, which has an insulated shaft, fixed exposed tip length, and a tip which has a sharpened trocar shape for self-penetration into the body.
The GSK Electrode is used for cingulum lesion destruction in the brain. The cingulum is known a priori to have typical dimensions, and thus the tip exposure may be set beforehand to correspond to general knowledge of the cingulum tip dimensions. However, never in the use of the GSK Electrode is it anticipated to do imaging studies of the anatomy to determine the size of a tumor or other target structure, and then set the tip exposure of the rf electrode to correspond to the dimension of the tumor from a given probe tract angle based on that imaging information. Thus, the intention of the GSK variable tip electrode was entirely different from the intention of the method in the present patent.
The sharp, trocar pointed RRE Electrode of Radionics was used for self-penetration of the tissue and placement of the electrode near the facet joints of the spine to cause heating of the neuro-musculature of the spine for pain relief. Nowhere was there ever the intention of using such a self-penetrating electrode for the treatment of cancerous tumors, nor was there any intention to devise a pointed electrode which has variable tip exposure to optimize the treatment of cancerous tumors. Thus, the method and design of the above-mentioned Radionics electrodes was distinctly different from that of the present invention. Another quantitative distinction between the GSK Electrode and the RRE Electrode with respect to the present invention is the size and length of the tips involved. Functional and pain lesions are typically never larger in dimension than approximately 10 mm in width and 10 to 14 mm in length. Thus, the tip exposure of the GSK variable tip electrode is limited to approximately 12 mm, and that of the RRE Electrode is limited to 7 mm. Furthermore, the diameter of these electrodes is 2 mm or less. With such limited tip sizes, limitation on lesion destruction volumes is reached. For the destruction of cancerous tumors, where the tumor volumes can be substantially greater than those just mentioned, much longer tip exposures would be required, and larger electrode tip diameters would also be required.